In ARDS, bacterial pathogens damage host cells, activate innate immune responses, and create a pro-oxidant environment leading to cell death via one of the death programs. This Project will focus on a recently described death program, ferroptosis, realized via Fe-dependent activation of lipid peroxidation under conditions of deficiency of glutathione peroxidase 4 (GPX4), a seleno-enzyme uniquely capable of reducing phospholipid hydroperoxides. We identified 15-hydroperoxy-arachidonoyl-phosphatidylethanolamines (15-HOO-AA-PE) as specific lipid biomarkers of ferroptosis. We also discovered that complexes of 15-lipoxygenase (15LOX) with a scaffold protein, PEBP1, play the major role in generating 15-HOO-AA-PE signals. We divulged ferroptosis as a death program of the human pulmonary epithelium. Ferroptosis occurs in alternatively activated macrophages with low levels of NO?/iNOS, thus causing immuno-suppression. Unexpectedly, we discovered that a common Gram-negative pathogen, P. aeruginosa ? that does not contain polyunsaturated phospholipid oxidation substrates? expresses 15LOX (pLoxA) which oxidizes host polyunsaturated PE, generates 15-HOO-AA-PE and causes ferroptosis in epithelial cells and macrophages independently of the endogenous host 15-LOX. Ferroptosis-inducing pLoxA was detected in clinical P. aeruginosa isolates from ARDS patients. 15-HOO-AA- PE were identified in the lung samples from severely immuno-compromised patients with ARDS. Thus, we postulate the existence of a vicious cycle whereby inflammation/oxidative stress driven ferroptosis supported by endogenous 15-LOX acts as the major contributor to immunosuppression that sets the stage for the secondary P. aeruginosa infection of immune-impaired lung and further enhancement of ferroptosis by exogenous bacterial pLoxA. We propose to design and use selective small molecule pLoxA inhibitors, which will act as anti-ferroptotic agents thus representing new classes of pulmonary protectors. Aim 1 will reveal and decipher pathogenic mechanisms through which reactions of phospholipid peroxidation catalyzed by isoforms of endogenous mammalian 15-LOX or exogenous bacterial pLoxA ? in conditions of GPX4/GSH deficiency ? lead to accumulation of hydroperoxy-phospholipids in murine lung epithelial cells (MLE) and alveolar macrophages and establish molecular identity and ferroptotic properties of these products. By using redox lipidomics we will identify and quantify 15-HOO-AA-PE biomarkers of ferroptosis in vivo using a two-hit model of immunosuppression. We will also employ iNOS KO animals exposed to P. aeruginosa to reveal the role of NO? as a regulator of pLoxA-driven AA-PE oxidation and ferroptotic death in the mouse lung vivo. In Aim 2, we will design and develop selective inhibitors of pLoxA regulating P. aeruginosa-driven ferroptosis in epithelia and macrophages as a new class of small-molecule cytoprotective agents preventing breach of the barrier and immunosuppression.